This invention relates to a process for recovering, in situ, oil from an oil shale deposit.
Oil shale deposits in Colorado and Wyoming have been well known for over fifty years. The Green River Formation, covering an area of approximately seventeen thousand square miles in south-western Wyoming, north-eastern Utah, and north-western Colorado, has oil shale deposits with total oil resources estimated to be eight trillion barrels of oil, in oil shales containing over ten gallons of oil per ton. The Piceance Creek Basin alone, in Colorado, has deposits containing 1.2 trillion barrels of oil in oil shales having oil content of over fifteen gallons per ton. The amount of oil in this formation alone is suifficient to supply the United States with oil for approximately one hundred ninety years, assuming a consumption of seventeen million barrels per day.
However, recovery of the petroleum from these enormous deposits has never been economical. Even after the huge recent increases in oil prices on the world market, the projected costs for recovery from this oil shale has remained higher than the costs of purchasing the oil in the world market.
The present invention is applicable to oil shales having varied amounts of oil content and covered by overburden. It can be applied to oil shales without recoverable valuable mineral content other than oil. It can also be applied to oil shales containing other minerals, where the recovery of those minerals is to be accomplished separately or where their recovery is not desired.
An object of this invention is to provide an economical process for producing oil from oil shale deposits.
The prior-art process of above-ground retorting comprises mining the oil shale, crushing the oil shale, heating the crushed shale in large ovens or retorts, recovering its petroleum values, cooling the spent shale, and finally disposing of the retorted spent shale residue.
It has been proposed to mine the oil shale by excavating large underground cavities or rooms, leaving supporting columns or pillars of shale between the rooms. Since, in this room-and-pillar method, the pillars must remain forever underground, their mineral values cannot be utilized, and only about 55-75% of the shale of the total shale present could be mined, leaving a loss of 30-45% of the shale, along with its oil and mineral content.
It has also been proposed to employ pit mining, first removing or stripping off the overburden and then mining the oil shale. Tremendous land scars result from this process, for the pits would be several thousand feet in diameter and up to three thousand feet deep. Current estimates are that open pit mining would become economical in the foreseeable future only for shales containing over twenty gallons per ton of oil.
In both pit mining and room-and-pillar mining, the shale would have to be transported, crushed, and screened. These process steps would be quite expensive and would consume large amounts of energy. Moreover, the construction and operation of above-ground retorts is expensive. Still further, the residue of the retorting, the spent shale, has to be disposed of. The quantity of this residue, depending on the oil content of the shale, is approximately 80-90% of the weight of the mined shale.
For example, for a plant to produce one-million-barrels of petroleum per day, the quantity of the shale which would have to be retorted (assuming 100% recovery of the oil, and even assuming 30 gallons of oil per ton of shale) would be 1.4 million tons per day or 511 million tons per year. Mining these quantities of shale for above-ground processing would necessitate an approximate doubling of the total current undergound mining capacity of the U.S.A. Moreover, the residue, or spent shale, which in this example would be approximately 85% by weight of the shale, would be 1.19 million tons per day and 434 million tons per year. The disposal of such quantities would cause considerable problems. Moreover, not only is the space requirement very high, but there is a danger that the water-soluble mineral content of the spent shale would be leached out by rain and would contaminate the surface and subterranean waters.